1. Field of the Invention
The present invention relates to a method for manufacturing a dielectric thin film capacitor.
2. Description of the Related Art
A dielectric thin film capacitor used in a semiconductor device such as DRAM or a noise filter has a structure in which a substrate, a lower electrode, a dielectric thin film, and an upper electrode are stacked in order. The dielectric thin film can be deposited by a sputtering process, a CVD process (chemical vapor deposition), a MBE process (molecular beam epitaxy), a sol-gel process, or a MOD process (metalorganic decomposition). However, the sol-gel process and the MOD process are advantageous from the viewpoint of manufacturing cost.
In the sol-gel process or the MOD process for depositing the dielectric thin film, a raw material solution prepared by dissolving an organic compound used as a dielectric raw material in an organic solvent is coated and then heat-treated in an oxidizing atmosphere. Therefore, an non-oxidizable noble metal, for example, Pt, is often used for the lower electrode.
Although a Si substrate is generally used as the substrate of the dielectric thin film capacitor, a Si oxide layer is formed on the surface of the Si substrate because Si becomes oxidized when being allowed to stand in air.
When a dicing-cut process is performed after the dielectric thin film capacitor is formed, an adhesive layer must be formed between the substrate and the lower electrode in order to improve the low adhesion between the Si oxide layer on the surface of the substrate and the noble metal of the lower electrode. The adhesive layer often comprises Ti (refer to Japanese Unexamined Patent Application Publication No. 8-78636).
Japanese Unexamined Patent Application Publication No. 8-78636 discloses that the use of Ti for the adhesive layer has the problem of causing warping of the substrate due to oxidation of the Ti layer and thus causing cracking in the dielectric thin film. This publication also discloses that warping of the substrate can be suppressed by limiting the thickness of the Ti layer functioning as the adhesive layer.
However, another problem occurs even if the thickness of the Ti layer is limited. Namely, even a limited Ti layer is inevitably oxidized in an annealing process or the like. When Ti is oxidized and diffuses at the interface between the lower electrode and the dielectric thin film, two additional problems described below occur.
First, when barium titanate (BaTiO3), strontium titanate (SrTiO3) or strontium barium titanate ((Ba,Sr)TiO3, abbreviated to “BST” hereinafter) is used for the dielectric thin film, the Ti oxide diffusing at the interface between the dielectric thin film and the lower electrode causes a crystal structure deviation at the interface which causes disturbance in crystallinity of the dielectric thin film. When the crystallinity of the dielectric thin film is disturbed, a sufficient dielectric constant cannot be obtained.
Second, the Ti oxide has a high resistance and a low dielectric constant and thus creates a state in which a layer having a high resistance and a low dielectric constant is present at the interface between the lower electrode and the dielectric thin film to significantly degrade the characteristics of the capacitor.
The above document further discloses that the dielectric thin film is preliminarily burned at a temperature lower than the crystallization temperature to planarize its surface, and then the upper electrode is formed on the dielectric thin film, followed by final burning at a temperature higher than the crystallization temperature. As shown in FIG. 2 of the document, the dielectric constant of the dielectric thin film formed by the production method is about 220, which is relatively low for BST. Under the present situation in which circuit integration is strongly demanded, a capacitor capable of achieving a high capacity with a small area is required, and thus a dielectric thin film having a higher dielectric constant must be formed.
While, the technique disclosed in the above document is capable of suppressing the occurrence of cracking, which causes a leakage current, to some extent, it has the problem of impairing the characteristics of the capacitor and causing difficulty in achieving a high dielectric constant.